Primordial germ cells (PGCs) are the embryonic precursors of the gametes of the adult animal. They can give rise to pluripotent stem cells termed embryonal carcinoma (EC) cells during the formation of testicular tumors. PGCs can also give rise to pluripotent stem cells termed embryonal germ (EG) cells when cultured in the presence of three polypeptide growth factors. The conversion of a PGC to a stem cell in vitro occurs in a matter of days. However, the alterations in gene expression that occur in PGCs during this period are unknown. This proposal is designed to test the feasibility of using suppression subtractive hybridization to identify the molecular changes that occur during the transition from a PGC to a pluripotent stem cell using mice as a model system. Identifying the molecular changes that occur during this process will generate new information about the mechanisms controlling i) PGC differentiation during gametogenesis, ii) the processes involved in PGC transformation and iii) developmental potency in mammals. The knowledge of these processes will aid in our understanding of the factors controlling fertility in mammals.